Such an appearance can be achieved by causing shear of the beverage. This encourages the liberation of small bubbles from the beverage and these gradually separate out to form the close-knit creamy head. It is well known that shear of the beverage can be caused by jetting fluid into the beverage in the container.
Various methods have been disclosed for jetting fluid into a beverage in a container upon opening of the container to cause shear of the beverage. GB-A-1,266,351 discloses a container which includes an inner secondary chamber which is pre-pressurised with gas. The chamber is initially sealed with a soluble plug which dissolves shortly after filling the container with beverage, when the pressure in the container is similar to that in the secondary chamber. A small orifice is included in the secondary chamber, and fluid is jetted from the secondary chamber via the orifice into the main body of the container causing the liberation of the required small bubbles in the beverage.
GB-A-2,183,592 discloses a container including a separate hollow insert having an orifice in its side wall. As the container is filled, beverage is introduced into the hollow insert through the orifice. Upon opening the container, beverage from the insert is jetted through the orifice into the beverage in the container again causing shear of the beverage.
WO-A-91/07326 discloses a system in which an insert which jets gas only into the beverage in the main body of the container is pre-pressurized with gas and includes closure means. The closure means remains sealed before filling and during the container filling operation but when the beverage container is subsequently opened, de-pressurization of the beverage container results in the insert releasing a surge of gas from a restricted orifice into the beverage to “seed” the required nucleation of dissolved gas bubbles to produce the required rich creamy foam. Since the insert is sealed at all material times before the container is finally opened by the consumer the container and insert combination can be filled as easily, simply and quickly as conventional container. Examples of the closure means includes a burst disc and a pressure responsive valve. A disadvantage of this type of system is that the insert may contain a residual pressure after the container has been emptied. There is a risk a consumer will cut open the empty container and thus be able to interfere with a pressurised insert.
WO-A-91/07326 discloses a very large number of ways in which the pressurized gas insert can be formed and mounted within the beverage container. In most examples, the insert is mounted so that, in use, it is located at a fixed position. However, an example is also described where the insert floats in the liquid in the container.
A problem which occurs with fixed inserts results from the way in which a container is handled during opening. When opening a bottle with a crown cork type closure the bottle is often tipped almost horizontally if opened using a fixed opener. Equally when opening an easy open feature, either a ring pull or a stay-on-tab on a can it is common to tilt the can on opening. In both cases, immediately after opening the closure the container is then tipped to dispense its contents. These actions can result in the restricted orifice of the insert not being immersed in the beverage whilst gas is being jetted from it. In such a case the insert does not function correctly.
GB-A-2280887 discloses a carbonated beverage container including a floating hollow insert having a first duckbill valve arranged to allow gas to enter the insert, and a second duckbill valve arranged to allow gas to be jetted from the insert. The insert is arranged to float on the beverage with the first valve in a headspace above the beverage, and with the second valve below the surface of the beverage.
The insert of GB-A-2280887 does not have to be pre-pressurized. As the insert floats on the beverage, the insert may be dropped into the container before or after filling, and therefore the assembly of the container and insert is much simpler than for containers in which the insert is fixed in the container. As the insert floats, the problems of orientation, including gas not being jetted into the beverage, and beverage entering the insert, which are associated with fixed inserts, are overcome. Further, the nature of the containers is not critical since it is not necessary to form an interference fit with them, or adapt them specifically to hold the insert at a particular location.
The use of duckbill valves through which fluid is jetted in the insert of GB-A-2280887 is particularly beneficial. The size of the aperture through which the fluid is jetted varies with the pressure difference across the valve and the nature of the fluid being jetted. This variation in the size of the aperture ensures the fluid jetting into the beverage causes optimum shear. This allows the volume of fluid required for jetting into the beverage to be reduced when compared to the volume required when jetting through a fixed size orifice.
The insert of GB-A-2280887 may be moulded from a plastics material such as polypropylene, or be formed of metal such as lacquered aluminium, lacquered tin plate, polymer coated aluminium, polymer coated tin plate or tin free steel. The duckbill valves are manufactured from a thermoplastic elastomer (TPE), for example a styrene-ethylene-butylene-styrene block co-polymer, and are mounted in holes in the wall of the insert. This complicates assembly of the insert and there is a danger that the valves may become separated from the insert and be swallowed. Furthermore, manufacture of duckbill valves from TPE is problematic, as described in our earlier specification GB-A-2292708. As TPE is elastic, the slit in a TPE duckbill valve cannot be formed by the usual method of mechanical splitting to form a brittle fracture. GB-A-2292708 describes a method of manufacturing TPE duckbill valves in which the slit is formed by fluid pressure.